VFB COMPMAX X 3 V TX2 1/13 October 2016 RED2541 Datasheet Rev01 Preliminary RED2541 LLC Controller for CV LED Drivers Features Advanced LLC Controller IC for high efficiency low-cost

www.redisem.com 1/13 October 2016 RED2541 Datasheet Rev01 Preliminary

RED2541

LLC Controller for CV LED Drivers

Features ■ Advanced LLC Controller IC for high

efficiency low-cost Constant voltage (CV) LED Drivers

■ Resonant half-bridge for excellent EMI using robust low-cost bipolar transistors

■ Optimised for optocoupler-based CV applications using secondary sensing

■ 50% duty cycle, variable frequency control of resonant half-bridge

■ +/-5% accurate limiting of output current using primary sensing regulation (PSR)

■ Boost current feature to ensures reliable startup with constant power loads

■ Protection modes:

Overload output including short-circuit

No-Load output

Over-temperature (internal or NTC)

■ Low standby power

■ Small SO8 IC package

Applications ■ Passive PFC CV LED drivers 20W - 100W

■ Active PFC CV LED drivers 40W - 300W

Order code

Part Number Package Packaging

RED2541AD-TR13 SO8 Tape and reel

Figure 1: Block diagram

Limits

VCOMPMAX +

-

RC

CSRC RESETVCOMPMIN

ISTART

DriverLogic

ISTART

TX1

TX2

CLK

SLEEP

POR

3.45VShunt

Regulator

Oscillator

RUN

VDD

GND

-

+

VFBAnalogControl

Startup

VCCLIM

VREF

SLEEP

VDD

Current averaging

CV Amplifier

-

+

InternalOver Temperature

Protection

IOTP

SH

UT

DO

WN

STARTUP

FA

ULT

S

FaultLogic

Max/Min Limits

INTC

-

+VNTC

VDD

NTC

NTC Shutdown

X 3

Standby

SO8

RED2541 LED LLC Controller


Device Pins

Figure 2: SO8 pin connections (top view)

Pin Functions

Pin # Name Function

1 VFB Feedback input for output voltage regulation. Connect to optocoupler feedback.

2 NTC Shutdown pin that can be used for over-temperature protection with an external NTC resistor. A voltage of <VNTC will shut the IC down. Pin also includes a current source.

3 TX2 Output to control transformer.

4 TX1 Output to control transformer.

5 RC External RC network sets the minimum [full power] switching frequency.

6 VDD IC Power Supply pin – nominally 3.45V

7 GND Chip ground.

8 CS PSR Current Sense input provides output current regulation and cycle-by-cycle over-current protection. The CS pin is connected to the half-bridge current sense resistor

1

2

3

4 5

6

7

8

GNDNTC

CS

VDD

RC

TX2

TX1

VFB



Typical Application

Figure 3: Typical Application Schematic: LLC converter with RED2541 CV controller

FeaturesRED2541 is an advanced CMOS control IC for resonant converters such as LC, LLC and LCC. The RED2541 control scheme accurately controls the driver output voltage by means of secondary side feedback employing an amplifier and optocoupler. The IC also accurately controls the driver output current with RediSem’s PSR technique, reducing cost, complexity and power consumption.

RED2541 uses the CSOC (Controlled Self-Oscillating Converter) scheme to drive two low-cost bipolar transistors in a half-bridge configuration. RED2541 is optimized to work with RediSem’s resonant converter topology with integrated Power Factor Correction.

The RED2541 PSR scheme regulates the LED drive current by modulating the converter frequency. Primary side current control enables very accurate overload regulation and detection.

The IC enters a controlled burst-mode operation at light load, minimising the converter's input power consumption. The point at which the IC enters burst mode is pre-set to a value derived from the primary-side current to keep the output voltage from rising and to keep the power consumption low.

Protection Features

The IC is able to detect a number of faults that cause the IC to enter a fault mode:

Output open circuit (no LED connected)

Output overload

Output short circuit

Over-temperature fault

During these fault conditions, the IC will continually attempt to re-start. Depending on the fault, there can be 8 dummy re-starts between re-start attempts when the IC re-starts while the converter remains off.

Ls

Lm

Cr

OutputT1a

T1b

T1c

T1d

T2

D1

D2

Cout

+

_

HT+

HT-

RED2541

NTCTX2

RC VDD

TX1

CS

VFB

GND

1234

5 8

Raux

Caux

Daux

Q1

Q2

Rcs

Rrc

Cdd

U1

T1

Crc

6

Rcs2 Rfb2

7

VOUT

VOUT

Rfb1

RNTC

VOUT

OptoRfb3



If the output is short-circuit, the auxiliary power to the IC fails and the IC shuts down. The IC detects this and when it next re-starts, it does so at half output current. It continues to do so until the short has been removed. If the fault is removed, the IC will automatically return to full output current.

The IC also has an instantaneous cycle-by-cycle over-current protection (OCP) level that will terminate any cycle instantaneously should the current exceed a pre-set level.

Over-temperature Protection & Shutdown

RED2541 has an NTC shutdown pin that can be used either in conjunction with a thermistor or with a logic signal. A high NTC resistance on the pin will allow the converter to run and a low NTC resistance will turn it off. Current is sourced from the NTC pin to allow simple connection to a thermistor. A small amount of hysteresis is also applied to this pin.

An internal over-temperature protection shuts down the controller if the IC temperature exceeds

125°C. The IC will restart the converter when the IC temperature drops by 8°C.

Automatic Dead-Time Control

An important feature of the Controlled Self Oscillating Converter is that the dead-time is controlled naturally. Unlike MOSFET half-bridge converters, it is not necessary to program the dead-time on RED2541. The bipolar switching transistors are turned on correctly through the self-oscillation of the converter and turned off by RED2541. This greatly simplifies the design process and improves the robustness of the LLC converter.

Capacitive Mode Protection

RED2541 includes a capacitive mode protection feature which prevents the converter from entering capacitive switching mode on a cycle-by-cycle basis by limiting the minimum frequency. This always ensures the Controlled Self Oscillating Converter continues to oscillate correctly.



IC Operation

Startup, Shutdown and re-start Figure 4 shows typical startup waveforms for RED2541. In SLEEP mode the IDD current is approximately 8uA (IDDSLEEP). Once VDD reaches 3.7V (VDDSTART) the IC enters STARTUP mode. During the initial period of approximately 40ms (2048 cycles) Vdd is allowed to drop to 2.4V or rise to 3.6V. This gives time for the application to pull up the output voltage. After this the IC

enters RUN mode when the controlled Zener clamp inside the IC regulates the VDD voltage to 3.45V (VDDREG). The IC current is now approximately 0.7mA (IDDREG) plus any excess current required to clamp VDD to 3.45V. If VDD falls below 3.45V (VDDREG) the Zener clamp turns off and IDD reduces to 0.7mA (IDDREG) only. If VDD falls below 2.9V (VDDSLEEP), the IC enters SLEEP mode. In this condition IDD reduces to 8uA. (IDDSLEEP).

Figure 4: IC Start-up waveforms

Output stage A diagram of the output stage can be seen in Figure 5. To start the converter oscillating the RED2541 issues start pulses through the TX pins during the first two cycles. These start pulses are 800ns long (tTXSTART) and provide 28mA (ITXSTART) current pulses from both TX1 and TX2 pins. After this the converter self-oscillates and no longer needs start pulses to maintain oscillation. A low

on-state NMOS transistor is used to turn the bipolar transistors off. It is controlled by the oscillator off-time. The NMOS device is turned to pull TX pin low, which switches off the corresponding bipolar transistor in the power converter half-bridge.

Figure 5: Output Stage

TX1 /

TX2

VDD

Start

Pulse

ITXSTART

Clamp

Time

Drive Stage

TX

1

TX

2

TX

1

TX

2

TX

1

TX

2

TX

1

TX

2

TX Pin voltage

RC Pin voltage

CS Pin voltage

TX

1

TX

2

VDD voltage

IDD current

TX

1

TX

2

TX

1

TX

2

SLEEP MODE RUN MODE

TIME

VDDSTART

VDDREG

IDDRUN

IDDSLEEP

Startpulses

VDDSLEEP

SLEEP MODE

TX

2

TX

2

TX

1

TX

2

TX

1

Power Down

Sleep40ms (2048 cycles)

STARTUP MODE STARTUP MODE



Voltage and current RegulationThe converter’s output voltage and current are controlled by two independent amplifiers within the IC, as shown in figure 6. Inside the IC there are two separate control loops that control the converter output voltage (in normal operation) and output current (during pullup or overload). The RED2541 regulates the output current and voltage

by controlling the frequency. An Oscillator Control Voltage is fed into the oscillator comparator to give the desired operating frequency. Figure 6 shows how the two current and voltage error amplifiers and their compensation networks are configured for a primary regulated resonant converter.

Figure 6: Error Amplifier Circuits

Voltage Control The VFB pin of the RED2541 is internally connected to a x3 amplifier. The voltage on the VFB pin is compared to the reference voltage of 1.2V and the difference is multiplied by a factor of 3 and is then fed into the oscillator. This can be seen in figure 6 where the 1R and 3R resistors control the amplifier gain. There is also a small amount of high frequency roll off to minimise disturbance form high frequency noise. An external control loop using a secondary side reference (e.g. TL431) and an optocoupler is typically used to complete the control loop.

Light loads The IC has a burst mode that is used to lower the input power at light loads so that standby power is minimised. The standby point is usually set to 5-10% of rated load current. enters a controlled burst-mode operation at light load, by switching in

and out of STANDBY, thereby minimising the converter's input power consumption.

Standby Entry RED2541 will enter standby when either the IC reaches its maximum allowable operating frequency; or when a low load condition is detected. When the IC detects this condition, it goes into standby. In standby mode, the converter is held in the off condition, with the IC waiting for a standby exit signal

Standby Exit During standby, the output voltage will fall slightly. The control loop then demands more power by turning the optocoupler off slightly, thereby lowering the voltage on the VFB pin. When the IC senses this, the controller will exit out of standby and begin the start sequence. The converter will again deliver power to the load and the IC will remain out of standby for a while until the load falls again, when the IC re-enters standby.

-

+ CV Integrator

Oscillator Control Voltage

CC Integrator

VCCREG

Limits

VCOMPMIN

VREF

CFB

VFBV

DD

Feedback signal

-

+

-

+R

CFB

Rfb1

Rfb2VCOMPMAX

CCFB

CCOMP

PSRAverageCurrentCSAVG

Main Converter Primary Current

CS Resistor

100RCS

1R

3R



Figure 7: RED2541 Current protection and control circuits

PSR Current Control Current regulation is used during pullup and during overload conditions. Figure 7 shows the two current control methods used in the converter:

1. constant current (CC) regulation; 2. an instantaneous peak current limit

(OCP).

PSR Average current estimation

Shown in figure 7 the signal from the CS pin is divided into two different paths. The bottom path provides peak instantaneous over-current protection (OCP) while the PSR Average Current estimation block provides the current regulation (CC) information. The voltage on the CS pin is an AC signal biased around GND. Inside the PSR block this signal is processed to provide a voltage proportional to the average converter output current.

Constant Current Regulation

The CC regulation circuit is shown in Figure 7. CC operation is defined by an internally compensated

control loop. This provides a system response time of approximately 300us in a typical application. The average current regulation point, VCCREG is pre-set to 100mV, referred to the CS pin.

Over Current Protection

Over-Current Protection (OCP) is an instantaneous termination of the current oscillator cycle and the transistor on-time. When a peak voltage greater than 300mV (VOCPTHR) is sensed on the CS pin the OCP comparator terminates the current oscillator on-time cycle. The oscillator is reset and the off-time begins resulting in the bipolar transistors turning off and the half-bridge commutating. This is repeated in subsequent cycles whenever the CS voltage exceeds the threshold. However, in a correctly designed converter it should not be possible to trip OCP in normal operation.

PSR average current estimator

-

+

+

-

OCPComparator

Oscillator Reset

CC Integrator

VCCREG

VOCPTHR

To Oscillator Comparator

Main Converter Primary Current

CS Resistor

100R

CS Pin

CSAVG



OscillatorThe oscillator (see Figure 8) controls the period of a converter half-cycle. Internal to the IC is an oscillator comparator that compares the voltage on the RC pin to a control voltage. The RC pin has a saw tooth type waveform and the control voltage is inversely proportional to the required frequency.

The control voltage can vary from 0.3V to 2.35V, resulting in a maximum to minimum frequency ratio of nearly 7x for any input voltage.

The timing capacitor CRC may be chosen within the range 100 – 1000 pF. The recommended type is a 5% COG/NPO capacitor.

The oscillator timing resistor RRC may be connected to either VDD or to the rectified DC bus, VHT. If connected to VDD, the value of RRC may be calculated using following equation:

𝐹𝑀𝐼𝑁 =1

2 × (0.8 × 10−6 − 𝑅𝑅𝐶 × 𝐶𝑅𝐶 × ln (1 −2.35

3.45))

This equation gives the lowest possible operating frequency of the converter.

Figure 8: Oscillator circuit

Oscillator Feed-forward Compensation

The oscillator may optionally include feed-forward compensation. Feed-forward compensation is recommended to minimise the line frequency ripple on the output. To apply the feed-forward compensation, the oscillator pull-up resistor RRC is connected to the DC bulk supply VHT instead of VDD. The value may be calculated as a function of the DC bulk voltage using the following equation:

𝐹𝑀𝐼𝑁 =1

2 × (0.8 × 10−6 + 𝑅𝑅𝐶 × 𝐶𝑅𝐶 ×2.35

𝑉𝐻𝑇)

To assist feed-forward applications, a switch is provided which connects the VDD pin to the RC pin while the controller is in SLEEP. This allows the RRC resistor to pull up the VDD supply for start up.

+

-

Oscillator Comparator

Drive LogicRC

VHT

or VDD

SLEEP

RRC

CRC

RC Pin Discharge

To TX1 / TX2 Drivers

VDD

Oscillator Control Voltage



NTC pin RED2541 features an NTC pin to interface with an external over-temperature circuit as shown in figure 9. The NTC pin features a current source to bias an external circuit featuring a Negative Temperature Coefficient resistor (NTC). A comparator determines the NTC pin trip point. When the IC is in SLEEP mode the NTC pin is clamped to GND. The external NTC circuit should be biased so that under normal operation the resistance on the NTC pin is above the trip-point RNTC (VNTC/INTC). The NTC pin is blanked for approximately 5ms at IC start-up to avoid any spurious triggering while the

external NTC circuit is settling. When the converter heats up the NTC resistance decreases. After the NTC resistance drops below the trip-point RNTC the IC will shut the converter down and restart if the converter has cooled down and the NTC pin resistance has increased. With the appropriate selection and placement of the NTC circuit it is possible to achieve accurate over-temperature shutdown anywhere on the LED converter PCB.

Figure 9: RED2541 NTC pin circuit

SHUTDOWN

FA

ULT

S

FaultLogic

INTC

-

+VNTC

VDD

NTC

RNTC

CFILTER

SLEEP



ABSOLUTE MAXIMUM RATINGS CAUTION: Permanent damage may result if a device is subjected to operating conditions at or in excess of absolute maximum ratings.

Parameter Symbol Condition Min Max Unit

Supply voltage VDD SLEEP mode: self-limited by IC start-up (VDDSTART) -0.5 4.5 V

Supply voltage VDD RUN mode: Self-limited by internal shunt regulator -0.5 4.0 V

Supply current IDD 0 10 mA

Input/output voltages VIO -0.5 VDD +

0.5 V

Input/output currents IIO -10 10 mA

Junction temperature TJ TJ_MAX limited by OTP (TOTPS_MAX) -20 +135 °C

Storage temperature TP -20 +125 °C

Lead temperature TL Soldering, 10 s 260 °C

ESD withstand Human body model, JESD22-A114 2 kV

Capacitive Discharge Model 500 V

NORMAL OPERATING CONDITIONS Unless otherwise stated, electrical characteristics are defined over the range of normal operating conditions. Functionality and performance is not defined when a device is subjected to conditions outside this range and device reliability may be compromised.

Parameter Symbol Condition Min Typ Max Unit

Minimum supply current IDDMIN 0.8 1.0 1.2 mA

Junction temperature TJ -20 25 125 °C

ELECTRICAL CHARACTERISTICS Unless otherwise stated:

Min and Max electrical characteristics apply over normal operating conditions.

Typical electrical characteristics apply at TJ = TJ(TYP) and IDD = IDDREG(TYP).

The chip is operating in RUN mode.

Voltages are specified relative to the GND pin.

VDD Pin


Supply voltage

VDDSTART Enter RUN mode from SLEEP 3.2 3.7 4.2 V

VDDREG IDD< IDDSHUNT 3.25 3.45 3.65 V

VDDSLEEP To enter SLEEP mode 2.7 2.9 3.1 V

Supply current

IDDREG In RUN mode, VDD<VDDREG 0.7 0.8 mA

IDDSLEEP In SLEEP mode 8 12 µA

IDDSHUNT VDD shunt regulator max current 8 mA



VFB Pin


VFB threshold voltage VREF TJ= 0°C to 85°C, VDD=3.45V 1.15 1.20 1.25 V

VFB amplifier gain 3

CS Pin


Constant current regulation VCCREG DC CS signal. TJ= 0°C to 85°C 96 100 104 mV

Instantaneous over-current protection threshold

VOCPTHR 300 mV

RC Pin


External capacitor range CRC 100 1000 pF

Oscillator Frequency Variation

∆FRC/FRC* TJ= 0°C to 85°C, CRC=330pF, VDD=3.45V, min frequency

5 %

Oscillator reset time TRCRST 0.7 µs

NTC Pin


NTC pin threshold

RNTC

Trip fault:

NTC resistance highlow

(TJ= 25°C)

14.0 15.5 17.0 kΩ

∆RNTC* Fault recovery hysteresis:

NTC resistance lowhigh 2 kΩ

NTC pin temperature coefficient

θRNTC* NTC pin resistance temperature

coefficient* +0.145 % / °C

TX1, TX2 Pins


On-state resistance RTXON 1.0 1.5 Ω

TX pin clamp current ITXCLAMP* TX pin frequency >30kHz 800 mA

Start-pulse output current ITXSTART TX pin voltage 2V 28 mA

Start-pulse width TTXSTART 800 ns

Over-Temperature Protection (OTP)*


Over-Temperature Protection threshold

TOTPS At silicon junction 115 125 135 C

Over-Temperature Protection reset hysteresis

TOTP_HYS At silicon junction 8 C

*: not tested in production



PACKAGE INFORMATION

Package Dimensions

SO8N package dimensions are shown below. All units are in mm.

Available packages

Package type Part number Moisture Sensitivity Level (MSL) Packaging

SO8 RED2541AD-TR13 3 (JEDEC J-STD-020) Tape and reel

2500 / 13” reel

Package Marking

RediSem RED2541

WXYZ

SO8N top-side marking for RED2541 RED2511= Part Number WXYZ= Lot Code, e.g. AAAA, AAAB



Status The status of this Datasheet is shown in the footer.

Datasheet Status

Product Status

Definition

Preview In development The Datasheet contains target specifications relating to design and development of the described IC product.

Preliminary In qualification The Datasheet contains preliminary specifications relating to functionality and performance of the described IC product.

Production In production The Datasheet contains specifications relating to functionality and performance of the described IC product which are supported by testing during development and production.

Contact Details RediSem Ltd. 301-302 IC Development Centre No 6 Science Park West Avenue Hong Kong Science & Technology Park Shatin, New Territories Hong Kong Tel. +852 2607 4141 Fax. +852 2607 4140 Email: [email protected] Web: www.redisem.com

Disclaimer The product information provided herein is believed to be accurate and is provided on an “as is” basis. RediSem Ltd assumes no responsibility or liability for the direct or indirect consequences of use of the information in respect of any infringement of patents or other rights of third parties. RediSem Ltd does not grant any licence under its patent or intellectual property rights or the rights of other parties. Any application circuits described herein are for illustrative purposes only. Specifications are subject to change without notice. In respect of any application of the product described herein RediSem Ltd expressly disclaims all warranties of any kind, whether express or implied, including, but not limited to, the implied warranties of merchantability, fitness for a particular purpose and non-infringement of third party rights. No advice or information, whether oral or written, obtained from RediSem Ltd shall create any warranty of any kind. RediSem Ltd shall not be liable for any direct, indirect, incidental, special, consequential or exemplary damages, howsoever caused including but not limited to, damages for loss of profits, goodwill, use, data or other intangible losses. The products and circuits described herein are subject to the usage conditions and end application exclusions as outlined in RediSem Ltd Terms and Conditions of Sale. RediSem Ltd reserves the right to change specifications without notice. To obtain the most current product information available visit www.redisem.com or contact us at the address shown above.

Documents

VFB COMPMAX X 3 V TX2 1/13 October 2016 RED2541 Datasheet Rev01 Preliminary RED2541 LLC Controller for CV LED Drivers Features Advanced LLC Controller IC for high efficiency low-cost